Tumors are generally treated using resection, radiation therapy, and chemotherapy, or a combination of these methods. In radiation therapy, the aim of the treatment is to obtain a high local tumor dose while minimizing the harmful effects on the surrounding normal tissue. To this end, the energy or dose deposition is controlled to conform to the shape of the tumor to the extent possible. Recently, good therapy results have been obtained through irradiation with ions instead of photons since the energy or dose deposition as a function of the penetration depth has a sharp maximum (called Bragg peak). One such method is known as passive beam application, where the beam is shaped by a specially adapted collimator. Alternatively, however, it is also possible to focus the ion beam precisely and to scan the tumor three-dimensionally with a needle-fine beam, a so-called “pencil beam” (raster scanning method, spot scanning method, continuous scanning method). In the raster scanning method, the beam remains on at a grid position for a defined period of time and is kept on while it is shifted to the next grid position. In contrast, in the spot scanning method, the beam is switched off between grid positions. In the continuous scanning method, the beam is moved continuously over the grid positions without remaining stationary on them.
Aside from protons, other ions, especially carbon ions, are currently used. At times, neon ions are also employed. The use of these ions provides a higher relative biological effectiveness (RBE) for the inactivation of cells as compared to photons. Due to its dependence on the dose level, on the type of tissue and, above all, on the particle type and particle energy, the RBE in the deep tumor area is generally higher than in the entry channel, and thus provides an additional therapeutic benefit.
In recent years, the Applicant, in cooperation with the University of Heidelberg, the German Cancer Research Center and the Dresden-Rossendorf Research Center, has achieved considerable clinical success with raster scanning irradiation using carbon ions and dedicated radiation planning. The advantages of this method are the virtual elimination of the need for absorber materials to prevent the generation of secondary particles and, above all, the good conformity of the generated dose distributions as compared to passive beam application, especially proximal to the tumor.
Initially, the tumors mainly treated were tumors in the region of the cranial base and along the spinal column, whose motion can be reduced to a negligible minimum through stereotactic fixation. However, plans for a broader clinical application of the raster scanning method in various therapy centers envisage to irradiate also other tumors with ion beams, in particular carbon ion beams, using the raster scanning method. Tumors in the torso area of the body, however, are subject to more motion, especially due to the patient's breathing, which causes the entire rib cage and parts of the abdomen to move and change shape, and sometimes even because of the heartbeat of the patient. When moving tumors or, in general, moving target volumes are treated using a scanning method, one is faced with the challenge that this motion can have an adverse effect on the homogeneity of the energy deposition. Experiments with phantoms have shown that during application of a scanned beam, overdosage and underdosage may occur in the target volume, so that a simple enlargement of the target volume by the magnitude of the motion, as is employed in the case of passive beam application, does not allow optimal treatment.
In order to correct the influence of the motion during application of a scanned beam, at present, fractional irradiation making use of safety margins, multiple radiation (called “rescanning”), irradiation with motion-phase-dependent interruptions (called “gating”), motion-compensated irradiation with active beam adaptation (called “tracking”), or combinations of the aforementioned methods/techniques are being studied and used in pre-clinical trials. During motion-compensated irradiation with active beam adaptation (tracking), the beam position is continuously adjusted to the motion of the tumor. In this process, the lateral position of the beam with respect to the beam direction and the range of the particles are continuously adjusted to the motion of the tumor. In this connection, reference is made to the dissertation of S. O. Grötzinger “Volume Conformal Irradiation of Moving Target Volumes with scanned ion beams,” Technical University of Darmstadt, Germany, 2004, and to that of C. Bert, “Bestrahlungsplanung für bewegte Zielvolumina in der Tumortherapie mit gescanntem Kohlenstoffstrahl,” (Irradiation Planning for Moving Target Volumes in Tumor Therapy with a Scanned Ion Beam), Technical University of Darmstadt, Germany, 2006, both of which are hereby incorporated herein in their entirety by reference. In any case, the motion-compensated raster-scanned ion beam application is known per se to those skilled in the field of particle-beam tumor therapy.
Thus, if no countermeasures (“motion mitigation”) are taken, the treatment of moving tumors with a scanned particle beam can in principle lead to dose errors due to the interaction of dynamic irradiation and moving anatomy. Even if the aforementioned methods are used, such as active beam adaptation (tracking), the path of the particle beam in the tissue may change despite the adaptation of the beam, for example when the motion of the tumor cannot be described by a pure translation. In fact, this is often the case since a motion of, for example, the rib cage may include rotational components and/or tissue deformations. Although active beam adaptation (tracking) allows the position of the Bragg peak, and thus the major portion of the particle dose, to be moved to the anatomically correct position, the change in beam path results in a change in the dose contribution to the remaining tissue, especially in the proximal region; i.e., upstream of the grid position where the Bragg peak is present. This results in local underdosage and overdosage as compared to planned dose deposition, which may be disadvantageous. In treatment simulations based on measured lung tumor data, the inventors found that, without considering the described dose changes, the dose coverage is significantly poorer as compared to simulated irradiation of a hypothetically stationary lung tumor (static irradiation, where no dose changes occur).
German Patent Application DE 10 2005 063 220 A1 describes measures for improving the time course of an irradiation which, however, are capable of being further improved with respect to the problems mentioned above.